1. Field of the Invention
This invention relates to a pin tractor for feeding blank paper for use in various printers, typewriters, etc., which are used as output devices for electronic computers, especially for personal computers.
2. Description of the Prior Art
In various printers, typewriters, etc. to be used as output devices for electronic computers, etc., pin tractors which feed blank paper by engaging pins of an endless belt carrying pins with feed perforations at both edges of blank paper and by running the endless belt carrying pins, are generally used. Such pin tractors, as disclosed by U.S. Pat. No. 4,214,691 for example, are so designed that a presser member to press blank paper being fed is supported revolvably on a frame by pins, and opening and closing of the presser member is controlled by a coil spring interposed between the presser member and the frame. In other words, the pressing member is forced into opening direction or closing direction with a change point as boundary by the coil spring.
Such pin tractors as stated above, however, require more parts, such as four pins, a coil spring, etc. to make the presser member revolvable and consequently are higher in manufacturing cost and poor in composability. Moreover, they have such disadvantage that while the presser member is pressing blank paper as it is in closed condition, spring force is the weakest but on the other hand, while the pressing member is in opened condition, spring force is the strongest. This means that when the change of blank paper is finished and the presser member is restored to its original condition (in closed condition), the presser member closes vigorously due to strong restoring force of the coil spring and will break blank paper around the perforations of blank paper if the engagement between pin perforations of blank paper and pins of the belt is unstable. Also, there are cases where strong impact force takes place at the presser member when the latter was closed and finger tips are pinched by such strong impact force.
In view of the fact that most of the pin tractors as mentioned above have a presser member and a frame, both made of synthetic resin of high rigidity such as polycarbonate reinforced with glass fiber, the inventor has been led to conceive that such problems as mentioned above could be solved by dispensing with the coil spring by utilizing the elasticity of synthetic resin. However, it has been found that such a pin tractor with no coil spring raises new problems, for example, (1) the repetition of opening and closing of the presser member involves abrupt lowering of blank paper pressing force of the presser member, (2) abnormal sound offensive to the ear generates whenever the presser member is opened or closed, and so forth. On examination, it has been revealed that such problems are attributable to unsmooth sliding during the revolution of the presser member in relation to the frame but can be solved by good choice of materials.
As disclosed in U.S. Pat. No. 4,130,230 and No. 4,194,660, pin tractors were usually of such construction that the belts carrying pins are turned by a pair of driving and driven sprockets. However, with the spread of personal computers in recent years, printers, typewriters, etc. tend to be miniaturized and consequently compactification of pin tractors has been practised to turn and move a belt carrying pins as it is slid along a guide member provided at a tractor proper by the rotation of a driving sprocket (a driven sprocket is omitted) and thereby save the space of the tractor (for example, Japanese Utility Model Application Laying Open Gazette No. 57-135537).
However, in the pin tractor from which a driven sprocket has been omitted, such as mentioned above, a motor which is the power source of driving a driving sprocket is linked with the movement of a printing belt, driving of a platen roll, etc., for which high driving torque is required, but it is also required to miniaturize a motor to save space, for which a decrease of driving torque for the belt carrying pins is required. For this purpose, it has been practised to decrease the bending modulus of the belt by using a softer material for the belt carrying pins and to decrease the initial tension by lengthening the belt dimension to some extent. This, however, raises the problem of the lowering of paper feeding accuracy due to elongation of the belt or the problem of tooth skip on the driving pulley. A method of coating the guide surface of the frame with teflon having a low coefficient of friction, for example, is available but this involves wear of coating layer and higher manufacturing cost.
Furthermore, in the conventional pin tractor with a belt carrying pins which has a plurality of pins at a regular pitch on the surface of belt base and teeth at regular intervals on the back surface, it is required to engage the pins with feed perforations at both edges of blank paper being fed. Therefore, the belts carrying pins in pin tractors at both edges of blank paper must be turned synchronously, for which pins of the belts carrying pins of the pin tractors at both edges of blank paper must be positioned symmetrically. This requires attentiveness at the assembling process and stricter inspection at the inspecting process, in other words, composability is lowered.
In the light of the disadvantageous of the conventional pin tractor as mentioned above, in the pin tractors disclosed in U.S. Pat. No. 4,130,230 and U.S. Pat. No. 4,194,660 teeth of a driving sprocket are reduced by one piece or several pieces and teeth at the back side corresponding to the pins of a belt carrying pins are removed entirely and by engagement of both, positioning of the belt carrying pins is effected accurately and easily. However, under this arrangement the number of teeth of a driving sprocket which engage with the belt carrying pins is decreased and therefore turning torque which can be transmitted is reduced and in the case where the tension of the belt carrying pins is unsatisfactory, the problem of meandering of the belt will be raised.